Existing commercially available light-emitting diodes (LEDs), including solid-state LEDs, organic light-emitting diodes (OLEDs), polymer light-emitting diodes (PLEDs), and electroluminescent devices, can be manufactured to generate light of different chromaticity with high electro-optical conversion efficiency. Single-colour and white light-emitting diodes can be used, for example, in luminaries whose luminous flux output and chromaticity can be digitally controlled.
The luminous flux output and chromaticity of LEDs are generally dependent on LED junction temperature, dominant wavelength shifts, manufacturing tolerances (especially dominant wavelength and luminous intensity binning), and device ageing. These dependencies are further compounded by their use in luminaires, where ambient operating temperatures and power supply voltage fluctuations must also be taken into consideration.
LED-based luminaires may also be used in conjunction with other light sources, including incandescent, fluorescent, and high-intensity discharge lamps, as well as direct and diffuse daylight, where it is desirable to maintain constant ambient light levels. It may also be desirable to maintain constant chromaticity with respect to electric lighting or vary the chromaticity in response to changing daylight conditions.
The luminous flux output and chromaticity of LEDs and LED-based luminaries can be digitally controlled in a number of different ways. It is usually necessary however to employ optical feedback to ensure predictable and repeatable luminous flux output and chromaticity from the LEDs and luminaires.
Digital control of the time-averaged luminous flux output of LEDs is well-known. For example, PWM control of the LED drive current, wherein the time-averaged luminous intensity is typically linearly proportional to the PWM duty cycle, is disclosed in U.S. Pat. Nos. 4,090,189 and 3,787,752, Today, PWM is the preferred method for LED luminous intensity control in that it offers linear control over a range of three decades (1000:1) or more without suffering power losses through current-limiting resistors, uneven luminous intensities in LED arrays, and noticeable colour shifts as identified by A. Zukauskas, M. S. Schur, and R. Caska, 2002, Introduction to Solid-State Lighting, New York, N.Y., Wiley-Interscience, p. 136. The PWM signals used to control the LEDs are preferably generated in hardware by microcontrollers and associated peripheral hardware.
A disadvantage of digital control is that the human visual system is extraordinarily sensitive to small but sudden changes in perceived brightness of illuminated surfaces, as shown for example by M. Rea, Ed., 2000, IESNA Lighting Handbook, Ninth Edition, New York, N.Y., Illuminating Engineering Society of North America, p. 3-21. To avoid the perception of such changes, it is necessary for the digital controller to generate at least 1,000 evenly-spaced intervals of perceived brightness.
The relationship between perceived brightness B and measured illuminance E (which is directly proportional to the luminous flux output of a luminaire) is described by Stevens' Law as follows:B=aE0.5  (1)where a is a constant. This “square law” relationship (M. Rea, Ed., 2000, IESNA Lighting Handbook, Ninth Edition, New York, N.Y., Illuminating Engineering Society of North America, p. 27-4) must be provided by the LED digital controller to provide perceptually smooth dimming of the LED or luminaire luminous flux output. This in turn requires the digital controller to generate at least 4,000 evenly-spaced intervals of luminous flux output. Given linear proportionality between the PWM duty factor cycle and LED luminous flux output, this requires a pulse width modulator with at least 12-bit resolution (4096 intervals), and preferably 14-bit (16,386 levels) resolution.
Unfortunately, most commercially available microcontrollers with integrated PWM modules provide a maximum of 10-bit resolution. (Some microcontrollers, such as the Philips LPC2132, provide PWM modules with up to 16-bit resolution, but with maximum PWM frequencies of only about 900 Hz.) Commercially-available PWM integrated circuits are available with 12-bit and greater resolution, but they are expensive in comparison with the microcontrollers they are designed to be connected to.
The ease with which the average luminous intensity of LEDs can be varied has led to the use of light sensors (which are typically silicon photodiodes with associated colour filters) to determine the relative luminous flux output from LEDs and the ability to feed this information back to the LED system via an optical feedback loop. This information can then be used to determine any changes required to the LED control signals in order to obtain the desired luminous flux output chromaticity when multiple sensors and various coloured LEDs are used.
As may be readily understood, a digital controller requires the analog sensor signal to be digitized for use in a feedback loop to control the luminous flux output of an LED or LED-based luminaire. In order to maintain feedback loop stability, it is necessary for the resolution of the digitized signal to equal or exceed that of the digital output signal to the LEDs. Unfortunately, commercially available microcontrollers with integrated analog-to-digital converter (ADC) modules provide a maximum of 10-bit resolution. Commercially-available ADC integrated circuits are available with 12-bit and greater resolution, but they are expensive in comparison with the microcontrollers they are designed to be connected to.
Another constraint is that the PWM frequency must be at least about 100 Hz to avoid the perception of visual flicker (A. Zukauskas, M. S. Schur, and R. Caska, 2002, Introduction to Solid-State Lighting, New York, N.Y., Wiley-Interscience, p. 136). With a PWM resolution of 12 bits, this requires a PWM module clock frequency of about 400 kHz. As noted by Pacurra, P., and R. Borras, “Microcontroller-based LED Drivers: Topologies and Trade-offs,” LEDs Magazine, October 2005, pp. 24-26, this is “almost impossible for a simple microcontroller” to achieve.
In addition, lifetime testing of LEDs has shown that continuous operation of LEDs using low frequency PWM with less than 100% duty factor can impose severe and repetitive thermal stresses on LED dies, which typically have thermal time constants of 10 milliseconds. At a PWM frequency of about 100 Hz, the luminous flux output can be severely degraded after only a few thousand hours of operation, and the mean-time-between failure (MTBF) can be reduced due to premature wire debonding and consequent device failure. At PWM frequencies above about 5000 Hz however, repetitive thermal stresses are substantially eliminated.
Another consideration is that LED power supplies include magnetic components such as transformers and inductors that are subject to magnetostriction and consequent acoustic vibration. If the power supplies are subjected to cyclical loads such as are presented by PWM-controlled LEDs, the power supplies may generate acoustic hum at the PWM frequency. If this frequency is within the range of human perception (about 20 Hz to about 20 kHz), it may be perceived as an annoying hum or squeal. If the PWM frequency is above this range, then acoustic hum will not present a problem. Unfortunately, a minimum PWM frequency of about 25 kHz and a PWM resolution of 12 bits require a PWM clock frequency of about 100 MHz, which is almost impossible to achieve with commercially available microcontrollers.
Power supplies for LED-based luminaires must also typically convert alternating-current mains power to direct constant current. This process generally results in a residual ripple current due to incomplete filtering, and whose magnitude is dependent upon the power supply load presented by the LED drive circuitry. If the filtering is insufficient, the luminous flux output of the LEDs may vary according to twice the AC line frequency and present visual flicker to an observer.
Yet another consideration is that the chromaticity of an LED-based luminaire must be held constant as its luminous flux output changes. This requirement can be difficult to achieve with luminaires having a plurality of colour LEDs, for example red, green, and blue LEDs or warm white, green, and blue LEDs, as the luminous flux outputs and dominant wavelengths of the LEDs are in general dependent on the LED junction temperatures, which are in turn dependent on the drive currents. These may result in a nonlinear and time-dependent relationship between LED drive currents and luminous flux outputs as the LED-based luminaire is dimmed, so that the chromaticity of the luminaire is interdependent upon the time-varying LED drive currents and junction temperatures.
Another consideration is feedback loop stability. PWM controllers in prior art systems typically use low-speed PWM frequencies and either sample-and-hold sensor signal sampling or large time-constant low pass filters for filtering the continuous sensor signals (which are pulse-width modulated square waves). Either approach results in slow feedback loops with typical response times of less than about 10 Hz. Consequently, observers may notice visual flicker or momentary chromaticity shifts when the luminous flux intensity is suddenly changed. These undesirable visual effects may persist for several seconds until the feedback loop settles to its new equilibrium state.
Finally, the sensors themselves may have temperature-dependent responsivities. While an optical feedback loop can compensate for changes in shifts in luminous flux output and chromaticity of LED-based luminaires with a plurality of colour LEDs or colour and white light LEDs, it cannot compensate for changes in sensor responsivities.
Luminous flux output control of LEDs has been addressed in a number of United States patents, for example U.S. Pat. No. 3,787,752 that describes intensity control for a light emitting diode display. The invention describes how a series of power pulses can be used to effectively control LEDs that are unmatched in their lighting characteristics for low electric currents but matched for electric currents near their optimal operating conditions. This document however, does not describe how the duty factors of the LED current pulses can be reproducibly and discretely set, and is also only defined as applied to displays.
U.S. Pat. No. 4,090,189 discloses another luminous flux output control circuit for LED displays. The invention describes a PWM method for controlling LEDs over a relatively wide range of brightness levels that extends stable operation into the lower brightness region. This disclosure also does not describe how the duty factors of the LED current pulses can be reproducibly and discretely set to control brightness of the LEDs with a desired resolution.
In S. Gage, M. Hodapp, D. Evans, and H. Sorensen, 1977, Optoelectronics Application Manual, New York, N.Y., McGraw-Hill Book Company, p. 5.10, a photoconductor controls the duty cycle of a monostable multivibrator that provides PWM control of the luminous flux output of a dot matrix LED array. Ambient light incident upon the photoconductor changes its resistance, and thereby provides a signal for an optical feedback loop. It is however an analog control system, and as such does not have a practical means to provide for example square law dimming, nor does it consider multiple interdependent colour channels.
U.S. Pat. No. 6,576,881 discloses a light output control system wherein a plurality of photodiode sensors with red, green, and blue colour filters are employed to repetitively sample the respective luminous flux outputs of red, green, and blue LEDs, and to generate control signals for a digital controller used to control the LEDs. However, it does not address the issues of sensor and digital controller resolution or PWM frequency. It also does not consider the need to sample the sensors at a sufficiently high rate such that the luminaire chromaticity remains perceptually constant when its luminous flux output is suddenly changed, and it requires complex mathematical calculations to convert the sensor inputs to LED driver outputs that are difficult to implement in a high-speed digital feedback loop.
U.S. Pat. No. 6,507,159 discloses a method and system for controlling a red, green, and blue (RGB) LED-based luminaire which can track the tristimulus values of both feedback signals and reference signals whereby the LED drive currents driving the LEDs in the luminaire are adjusted in accordance with the deviations of the sensed tristimulus values and the reference tristimulus values until the deviations no longer exceed certain predefined maximal values. This invention utilizes photodiodes with filters and drives the LEDs at predetermined constant current amplitude. It does however not consider the need to sample the sensors at a sufficiently high rate such that the luminaire chromaticity remains perceptually constant when its luminous flux output is suddenly changed, and it requires complex mathematical calculations to convert the sensor inputs to LED driver outputs that are difficult to implement in a high-speed digital feedback loop.
U.S. Pat. No. 6,630,801 discloses a method and system for controlling an RGB based luminaire which measures the output of filtered and unfiltered photodiodes and correlates these values to chromaticity coordinates for each of the red, green, and blue LEDs of the luminaire, which is similar to that of U.S. Pat. No. 6,507,159, wherein U.S. Pat. No. 6,630,801 additionally utilizes an unfiltered photodiode as an improvement to the system. This system again however does not address the issues of sensor resolution and temperature dependencies. It also requires the use of memory-intensive lookup tables whose values must be predetermined for specific LEDs, and consequently it is not responsive to temperature-dependent changes in dominant wavelength or LED manufacturing tolerances.
U.S. Patent Application No. 2003/0230991 describes a similar a system that uses detected light signals fed back to lighting systems for control thereof, including the use of thermal sensors whose function is to provide signals for a thermal feedback circuit that compensates for LED luminous flux temperature dependencies. The invention does not however consider the issues of sensor digitization resolution.
U.S. Pat. No. 6,833,691 discloses a system and method for providing digital pulse width modulation. The invention describes a pulse width modulation system for use in a switching power supply circuit that provides high-resolution pulse width modulated signals. The system is configured to receive a control signal comprising a (m+n)-bit binary word and to provide a pulse width modulated signal with a predetermined average duty cycle having a resolution of substantially 2(m+n). The pulse width modulation system includes a timing circuit for providing 2m timing signals, a dithering circuit, and a signal generator. Upon receiving the control signal, the dithering circuit is configured to provide a modified control signal, which comprises a series of up to 2n m-bit binary words. The signal generator is configured to receive the timing signals and the modified control signal and to provide the pulse width modulated signal having a duty cycle, which, when averaged over 2n timing cycles, is approximately equal to the predetermined average duty cycle. The pulse width modulated signal is used by a switching power supply circuit to control at least one power switching device. In particular, this invention uses a complicated signal generating circuit with adders, delays, multiplexers, memory, and latch modules. In addition, in its preferred embodiment (m+n)-bit control words are mapped into a sequence of m-bit PWM duty factors in a way that creates artefacts when the (m+n)-bit word assumes its maximum value of 2m+n−1.
Therefore there is a need for a new feedback and control system for digitally controlled luminaries such that it can provide perceptually visual smooth dimming and chromaticity control overcoming the identified inadequacies of the prior art.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.